(1) Field of the Invention
The invention relates to processes for packaging MEMS devices, and MEMS packages produced using the method, and more particularly, to a method of packaging MEMS devices having cost-effective electrical interconnections and a sufficient acoustic chamber.
(2) Description of the Related Art
Micro-electro-mechanical systems (MEMS) devices are known which convert physical phenomena, such as pressure, acceleration, sound, or light, into electrical signals. Each type of MEMS device interacts with the world in a different way, and demands custom or at least semi-custom packaging solutions. So-called system-in-package techniques attempt to form an entire microsystem—which could include a microprocessor, communications components, actuators and sensors—within a single package. However, packaging of a MEMS device is totally different from packaging an integrated circuit device. MEMS devices are categorically different from ICs despite sharing some fundamental processing technologies. Packaging is the biggest challenge for commercializing most MEMS devices. The term “MEMS package” is used in this document to imply a package including at least one MEMS device.
The packaging requirements of a MEMS microphone or acoustic sensor are complex because the devices need to have an open path to receive sound pressure and yet the devices need to be protected from external environmental hazards like particles, dust, corrosive gases and humidity. In addition, the device must have an acoustic chamber to be functional. A sufficient acoustic chamber is an essential requirement to guarantee the desired performance of any microphone/acoustic sensor. An insufficient acoustic chamber will damp the diaphragm movement and increase the acoustic noise. In considerations of packaging a MEMS microphone, it is also desired to avoid expensive process, substrate, and packaging materials.
For example, FIG. 1 illustrates a MEMS package as disclosed in PCT patent application PCT/US2005/006565, assigned to Tessera. An opening for external energy is shown at the top of the figure over the MEMS device. This opening directly faces the external environment and could result in exposure of the MEMS device to environmental hazards such as particles, dust, corrosive gases, and humidity. Surface Mount Technology (SMT) pads for external connection 132 are shown. Isolated via connections 136 to the SMT pads cannot form a good seal.
FIGS. 2A and 2B illustrate two examples of a MEMS package as disclosed in U.S. Pat. No. 7,242,089 (Minervini). The acoustic chamber 18 is of insufficient size due to the constraint of package height since it is formed within the substrate. This patent also discloses an expensive multi-layer PCB to create the required back chamber under the MEMS sensing element.
U.S. Pat. No. 7,202,552 (Zhe et al) teaches methods of using flexible printed circuit boards and folding processes to make MEMS packages. However, the MEMS die may be exposed to external environmental hazards using this method. Co-pending U.S. patent application Ser. No. 11/333,579, filed on Jan. 17, 2006, and herein incorporated by reference in its entirety, discloses the use of a metal meshed hole for environmental protection, but the flexible printed circuit board and processing costs are still high.
Co-pending U.S. Patent Application 2007/0205492 (Wang Zhe), assigned to the same assignee as the present invention and herein incorporated by reference in its entirety, shows a hollow chamber between two PCB's underlying the MEMS device. An opening in the second PCB is not aligned with the MEMS device.